Semiconductor lasers utilizing internal saturable absorbers

ABSTRACT

Trapping centers are controllably introduced into a junction laser by diffusing a P region to within at least 1.5 Mu of the junction. The centers, which act as saturable absorbers, produce bistable regions of operation in c.w. junction lasers operating above the delay transition temperature, the laser being either on or off depending on its previous history of operation. Optical logic and memory devices, as well as methods for fabrication, are discussed.

' United States Patent Dyment et al. [451 Apr. 4, 1972 [54]SEMICONDUCTOR LASERS UTILIZING [56] References Cited INTERNAL SATURABLEABSORBERS OTHER PUBLICATIONS [72] inventors: John C. Dyment; Thomas L.Paoli, both of N I taL u A dph L a n J l 69 Chatham; Jose E. Ripper,North Plaine son e pp 1e yslcs 6 u y pp field an of Primary Examiner-RoyLake [73] Assignee: Bell Telephone Laboratories, Incorporated, AssistantExaminer-Darwin R. Hostetter r y k y ghts, NJ. Attorney-R. J. Guentherand Arthur J. Torsiglieri 22 F1 d: Dec. 1 1969 I 1 57] ABSTRACT [2]]Appl. No.: 881,185

Trapping centers are controllably introduced into 21 unction laser bydiffusing a P region to within at least 1.5;. of the junc- [52] 5 tion.The centers, which act as saturable absorbers, produce bistable regionsof operation in c.w. junction lasers operating [5 Cl "Hols 3/18 19/08 26 above the delay transition temperature, the laser being either 581Field of Search 331/945 307/312 depending its hiswry opti' cal logic andmemory devices, as well as methods for fabrication, are discussed.

13 Claims, 8 Drawing Figures PATENTEUIIPII 4 I972 0 EQUIVALENT LOSSCURRENT SHEET 1 [IF 3 q F/G. 24

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(PRIOR ART) OFF REGION ON REGION INJECTION CURRENT I INJECTION-CURRENT IJ. c. DYME/VT wvavmas T. L. PAOL/ J- E. R/PPER 8V H.

AT TOR/VF I PATENTEDAPR 4 I972 I 3,654,497

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SHEET 3 or 3 FIGS REGIONI REGION 2 READ LASER B 63 REGION 2 MEMORY LASERA FIG 68 READ LASER E REGION I/Z /READ LASER F REGION 2 F \MEMORY LASERD SEMICONDUCTOR LASERS UTILIZING INTERNAL SATURABLE ABSORBERS BACKGROUNDOF THE INVENTION This invention relates to a method for controllablyintroducing saturable absorption trapping centers into junction lasersand to bistable, memory and other optical devices utilizing suchcenters.

Bistable lasers have obvious application in optical communicationssystems in general as logic or memory devices. There are at present,however, few bistable injection lasers known in the prior art. One ofthe few is termed the double diode, a single conventional, uniformlydoped P-N junction diode provided with a pair of separate contacts onthe P-side and a single contact on the N-side. The device, as describedin U.S. Pat. No. 3,427,563 issued on Feb. 11, 1969 to G. J. Lasher,operates on a pulsed basis and is driven by a pair of separate sourcesconnected to the separate contacts to produce I and 1 through adjacentregions 1 and 2 of the diode. I is maintained below a first thresholdand hence region 1 of the diode is absorptive (i.e., no populationinversion is established) and represents loss to the laser radiation. Iis increased above a second threshold (which includes the loss fromregion 1) causing the loss in region I to saturate and effectivelyreducing the total loss. Now, the device continues to lase even though I2 is reduced below the second threshold.

The double diode is disadvantageous for at least two reasons, however.First, the need for separate sources and separate contacts on the P-sideincreases the complexity of the fabrication process with its attendanthigher cost. Secondly, the device has been operated on a pulsed basisonly. In order to operate continuously (C.W.) a heat sink would have tobe provided, generally, on the P-side in order to take advantage of theproximity of the junction. This would necessitate moving the pair ofseparate contacts to the N-side and would probably be detrimental tobistability, due to current spreading in the thicker N-region.

The fabrication of the double diode is described in the aforementionedpatent, and more specifically in an article by G. J. Lasher and othersin Journal of Applied Physics, 35, 473(1965). The procedure followedinvolved conventional diffusion techniques to form P" and P regions inwhich the junction depth was 25p. and the P*-P interface depth was 22p.(see Journal of Applied Physics, supra at 474). The P and P regions wereformed, however, in a single diffusion, and, as indicated above, theseparation between the junction and the PP interface was 3.0 1..

In other lasers fabricated by us, but not in accordance with the presentinvention, the P' layer was typically Ody-0.3;; deep and the junctiondepth was much more than 2.011., thus making the separation between thejunction and the PP interface much more than l.5p.. The thin P layer wasused primarily for making good ohmic contact to the diode. As will bedescribed hereinafter, such structures do not introduce a sufficientnumber of trapping centers to produce saturable absorption.

It is, therefore, a broad object of the present invention tocontrollably introduce saturable absorption trapping centers into ajunction laser.

It is also an object of the present invention to produce bistableoperation in a semiconductor injection laser.

It is another object of the invention to produce such stability in alaser operating on a C.W. basis.

It is still another object of the invention to produce such bistabilityby means of saturable absorption.

It is yet another object of the invention to produce such bistabilitywithout the need for separate contacts and separate sources to controlthe saturable absorption.

SUMMARY OF THE INVENTION These and other objects are accomplished inaccordance with an illustrative embodiment of the invention in whichtrapping centers are introduced near the junction region of a C.W.semiconductor injection laser operating above its delay transitiontemperature. The centers act as saturable absorbers and produce bistableregions of laser operation in which the laser is either on or offdepending on its previous history of operation. The trapping centers areintroduced by diffusing a P region into the P-side of the laser diode toa depth such that the separation between the junction and the I-"Pinterface is less than 1.5a. Moreover, in the structure of the presentinvention, the junction is shallow (e.g., the P layer is 1.0p. deep withthe junction depth being about 1.8a). This structure is essential forthe trapping centers to be saturated by the optical field in thejunction region, as will be described more fully hereinafter.

Before discussing the invention in detail, a brief description of thedouble acceptor trap theory of saturable absorption in trapping centersis in order. As described in Time Delays and Q-switching in JunctionLasers-I-Theory, J. E. Ripper, IEEE, Journal ofQuamum Electronics, QE-S,391, (Aug, 1969), this theory is based on a trapping center that existsin three states depending on the number of electrons it contains. Whenin its nonabsorbing first state, the trapping center (or trap) cancapture one electron whose energy is near the energy of the valence bandedge and thus enter its optically absorbing second state. In the secondstate, it can capture another electron whose energy is near the energyof the conduction band edge and thus enter its nonabsorbing third state.The second electron can be captured in two ways, either directly fromthe conduction band or from the valence band with the absorption of aphoton accounting for the energy difference. The latter mechanismproduces optical loss which not only accounts for time delays observedin pulsed junction lasers, but also, when saturated by the optical fieldinternal to the laser, produces the bistability herein described whenthe laser is operated C.W. and above its transition temperature. It istherefore important to note that if the junction temperature were lessthan the transition temperature, the traps would be transparent tooptical radiation and would neither act as saturable absorbers norproduce bistability. Consequently, it is desirable to reduce thetransition temperature, as by means of the deep I region aforementioned,in order to insure that the junction temperature (for C.W. operation)can be maintained above the transition temperature.

The transition temperature in junction diode lasers is defined in anarticle in IEEE, Journal of Quantum Electronics, QE-4, 155 (1968) by J.C. Dyment and J. E. Ripper. As described therein, when a current pulseis applied to a conventional laser, either normal lasing or spontaneousemission is observed. In normal lasing, stimulated emission occurs aftera delay time t which can vary from a few nanoseconds to a few hundrednanoseconds, depending on the temperature, and

' usually continues for the remaining duration of the pump pulse. Inmost diode lasers, there is a temperature T,, termed the transitiontemperature, below which t is very short (=10- sec.) and above which Iis relatively long l0"sec.).

BRIEF DESCRIPTION OF THE DRAWING The objects of the invention, togetherwith its various features and advantages, can be more easily understoodfrom the following more detailed description taken in conjunction withthe accompanying drawings, in which:

FIG. 1 is a schematic of a bistable P-N junction laser in accordancewith an illustrative embodiment of the invention;

FIG. 2A is a graph of equivalent loss current versus injection currentfor a bistable laser in accordance with the invention;

FIG. 2B is a graph of equivalent loss current versus injection currentfor a conventional laser;

FIG. 3 is a graph of laser output power versus injection current inaccordance with the invention;

FIG. 4 is a graph of injection current versus heat sink temperature fora bistable laser in accordance with the invention;

FIG. 5 is a schematic of a second embodiment of the invention;

FIG. 6A is a schematic of a third embodiment of the invention'for use asa logic device; and

FIG. 6B is a schematic of a fourth embodiment of the invention for useas a logic device.

DETAILED DESCRIPTION Structure I An illustrative embodiment of abistable laser in accordance with the present invention is shown in FIG.1 as comprising a P-N junction diode 10 in which a deep P region 12 isformed in the P-side. The N-side is provided with a metallic contact 13and is bonded to a metallized heat sink l4 (illustratively a diamondheat sink coated with a metallic layer 16). On the P layer 12 isdeposited another metallic contact 18, preferably a stripe contact formode control as described in U. S. Pat. No. 3,363,195 of R. A. Fumanageand D. K. Wilson. Across the contacts are connected a bias source 20(e.g., a battery) in series with an AC switching source 22. The entirediode structure is typically surrounded by cooling apparatus (not shown)in order to control the temperature of the device.

Fabrication The diode 10 may be fabricated by making two successive Zndiffusions in an N-type GaAs substrate having a concentration of about 3X 10 electrons/cm. These diffusions produce the P and P+ layers shown inFIG. 1. First, the P-layer is formed by using a diffusion sourcecomprising a 2 percent solution of Zn in gallium saturated with undopedGaAs which creates a surface concentration of Zn acceptors of aboutl0/cm 800 C for 3 hours, produces a 1.8;. layer and forms the lasing P-Njunction. The more heavily doped P layer is formed by diffusing from apure ZnAs source for 65 minutes at 650 C. which creates a surfaceconcentration of Zn acceptors greater than l0 /cm layer is about 1.0Both diffusions are conveniently accomplished by the box method asdescribed by L. A. DAsaro in Solid State Electronics, 1, 3 1960). Afterthese diffusions, the fabrication proceeds in a standard manner toproduce stripe geometry metallic contacts.

The diode so constructed with a P layer of about 1.0a thicknessexhibited a transition temperature T, of about 150 K. and bistabilityfrom T, to well above room temperature. By way of contrast, aconventional diode (in which the second diffusion was for minutes at 650C., the P layer thickness was 0. l p.-0.3;i, and the transitiontemperature was about 300 K.) exhibited only normal lasing, notbistability. It has therefore been determined that it is preferable toutilize a shallow junction (e.g., 2.011. deep) in combination with adeep P layer (i.e., less than 1.5g. from the junction).

The deep P layer is effective in lowering T, not only because it tendsto confine injected electrons, but also because it increases the numberof trapping centers in the vicinity of the junction. It is these centersnear the junction which are saturated by the internal optical field toproduce bistable operation.

While the primary parameter which decreases T, is a small separation ofthe P layer from the junction, other factors also reduce T,, e.g.,lighter doping of the substrate, a special heat treatment, or a longertime for the first diffusion when using a weaker source to achieve thesame junction depth. All of these latter techniques are described in theaforementioned article by J. C. Dyment and J. E. Ripper in IEEE, Journalof Quantum Electronics, QE-4, 155 (1968).

Evidence that the increase in saturable absorption trapping centers andthe decrease in T, was caused by fabricating the P*- layer in accordancewith our invention is given by the following example. A single N-typesubstrate was diffused to form the junction at 2.1a. After this initialdiffusion the substrate was cut into four diodes D1 to D4 which wereprocessed as follows:

D1: Normal processing for 15 minutes at 650 C. to form a conventional,thin I layer about 0.1;; deep.

D2: Processing as for D1 but for 90 minutes at 650 C. to form a P* layerabout 1.0 deep in accordance with the invention.

D3: Heat treating for 90 minutes at 650 C. (as described in the .I. C.Dyment et al article, supra) without the diffusion source (Zn) andprocessing as for D1 to produce a thin P* layer about 0.1;. deep,

D4: Processing as for D3 but extending the heat treatment for 180minutes.

The lasers made from diodes D1, D3 and D4 all had very high transitiontemperatures of about 320 K. and exhibited small saturable absorption.On the other hand, lasers made from diode D2 in accordance with ourinvention had T, 230 K. with a large saturable absorption and aQ-switching region extending in some cases as high as 370 K. Thus weconclude that neither the shallow P layer,-nor such a layer combinedwith the heat treatment, will lower T, or produce saturable absorption.However, when the separation of the P-I interface and the junction isless than 1.5 4, T, is lowered and saturable absorption results.

OPERATION For the purpose of analysis, the laser gain G is assumed to beproportional to the injection current I: G(l) Bl, 1 where B is aconstant. The internal losses L(T,I) are given by the sum of twocomponents the normal laser loss which is exponential with junctiontemperature T, and the loss caused by the n traps in the absorbingsecond state:

form of the curve of FIG. 2A. For a constant heat sink temperature, I 1is a continuous function of I with a discontinuity in its firstderivative when 11 The condition for bistability is that for some valuesof l and T the laser can be stably on or off, thus requiring theexistence of two values of 11, one larger and one smaller than I. Morespecifically, with reference to FIG. 2A, as the injection current isincreased along curve 30 from zero, the laser is OFF (spontaneousemission;l and remains off until point B.

Further increases in current cause the laser to switch ON (stimulatedemission; point B; I 1 As the current is increased beyond point B, curve32 is followed. 0n the other hand, if the current is now reduced, thelaser remains ON until point A Further reduction in current causes thelaser to switch OFF to point A. The region between points A and B isunstable.

At high currents, where heating tends to quench lasing, a secondbistable region exists between 1 and I with the region between points Cand D being unstable. Thus, as the current is increased along curve 32,the laser remains ON until point D, where it switches OFF to point D oncurve 34. When reducing the current along curve 34, the laser remainsOFF until point C, where it switches ON to point C.

Note that in FIG. 2A the points A and D correspond to points where whichis an essential condition for bistability. By way of contrast, FIG. 2Bshows a graph of l I versus I for a conventional and monostable laser inwhich the transition temperature is relatively high, i.e., I region istoo shallow to produce saturable absorption trapping centers. Thefunction I (1 shown is single valued for all values of l, with lasingoccurring between points E and F. FIG. 2B is conspicuous for its absenceof points satisfying equation (4) and hence a laser exhibiting such acharacteristic is not bistable.

The output of a laser in the lasing region is proportional approximatelyto I I The behavior described with reference to FIGS. 2A and 2B pointsto a fundamental difference between bistable and normal lasers: theexistence of a discontinuity in the laser output power as the laserturns on or off, instead of the sharp but continuous increase in lightoutput at threshold in normal lasers. The discontinuous behavior of Ibistable lasers is shown in FIG. 3 where laser output power is plottedagainst injection current for a constant heat sink temperature. PointsA, A, B, and B again correspond to the points of FIG. 2A.

The bistable regions of operation are also shown in FIG. 4 whereinjection current is plotted against heat sink temperature for a stripegeometry laser having a low transition temperature of about 105 K. andmounted on a diamond heat sink. (See Continuous Operation of GaAsJunction Lasers on Diamond Heat Sinks at 200 l(., J. C. Dyment and L. A.DAsaro, App. Phys. Letters, 11, 292 (1967)). In region I the laser isalways ON, and in region 111 always OFF, regardless of the previoushistory of laser operation. In region 11, the bistable region, the laseris ON or OFF depending on whether it was last in region I or 111,respectively.

The bistable operation can be illustrated by following the constanttemperature T line in FIG. 4. As the current is increased from zero, thelaser turns ON at point B and OFF at D. As the current is decreased fromabove point D, it turns ON at point C and OFF at point A. The points A,B, C, and D correspond to those of FIG. 2A.

In the portion of region ll above T 123 K. for this particular diode,the laser cannot be turned ON by varying the current along a constantheat sink temperature line, but only by heating the diode along aconstant current line. Above T 126 K., the laser does not operatecontinuously.

LOGIC DEVICES The laser in accordance with the invention is readilyadapted to perform logic functions which do not rely upon bistability,but rather rely upon the changes in threshold caused by saturation ofthe trapping centers. For this purpose, it is often desirable that the Player, as shown in FIG. 5, be introduced into only a part (region 1) ofthe P-region. This type of fabrication is readily accomplished usingappropriate SiO layers (doped with phosphorous, for example) andphotolithographic techniques well known in the art. With saturableabsorption trapping centers only in region 1, and none in region 2, thisdevice can be electrically switched (by source 22, for example) both onor off by respectively saturating the losses in region 1 or the gain inregion 2.

In the logic devices shown in FIGS. 6A and 6B, trapping centers areintroduced, as described above, only in the crosshatched regions (region1 of FIG. 6B and regions 1 and 2 of FIG. 6A). These figures are topviews of the devices with the rectangular regions being provided withmetalliccontacts for connecting suitable bias and pump sources (notshown). Thus,

'each device comprises three independent lasers: a memory laser A or Dand a pair of read lasers B and C or E and F disposed so that theoptical output of the read lasers is directed to separate regions (1 and2) of the memory laser. The resonators of lasers A, B, and C are formedby surfaces 60-61, 62-63, 64-65, respectively, which are cleaved orpolished to be optically flat.

In the device of FIG. 6A, the current threshold of memory laser A(designated I with read lasers B and C OFF) can be lowered by an amountAI when one of the read lasers is turned ON because a part of thetrapping centers will be saturated (e.g., those in region 2 common tolasers A and B if read laser B is turned ON) by the external opticalfield of the read laser radiation. With both read lasers B and C turnedON, the threshold of memory laser A is about I ,,2AI. By applying tomemory laser A a current pulse of amplitude 1,, such that (I -AI) 1 I itwill lase (i.e., turn ON) if either read laser is ON, thus performing alogical OR function A=B+C. By applying a current pulse of amplitude Isuch that (I,,2AI) 1 (I -AI) memory laser A will lase only if both readlasers are ON, thus performing a logical AND function A 32 BC.

The device shown in FIG. 68 can be utilized as a nondestructive memorydevice. Note that the trapping centers (cross-hatched region 1) arelocated in only a portion of memory laser D. When memory laser D is ON,the threshold of read laser E is lowered by trap saturation and of laserF increased by gain saturation. By appropriately choosing the amplitudeof current pulses applied to read laser E, it will lase only when memorylaser D is ON, thus reading the state of laser D(E D). Similarly, readlaser F reads the logical negative of memory laser D, lasing only whenlaser D is OFF (i.e., F D).

The device of FIG. 6B cam also be used to perform other logic functions(e.g., D=E+F;D=E'F) in a manner analogous to that described withreference to the logic device of FIG. 6A. For example, assume trapsaturation lowers the threshold I by A1,; and gain saturation increasesit by A15. The function D=E+F is performed if the injection current l oflaser D is such that (I-Al,;) l, and l, is less than the smaller of Iand (I,,,K +AI,.-). Similarly, the function D= E-Fis performed if theinjection current I is such that I (I,,,+AI and 1 is greater than thelarger of I and (I,,,-AI +Al,-).

It is to be understood that the above-described arrangements are merelyillustrative of the many possible specific embodiments which can bedevised to represent application of the principles of the invention.Numerous and varied other arrangements can be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention. In particular, many devices can bedevised utilizing pulsed operation as well as CW. operation, the latterbeing preferred, however, for the embodiments disclosed herein.

What is claimed is:

1. An optical device comprising:

a continuous wave P-N junction laser having a characteristic delaytransition temperature,

means for maintaining the temperature of said junction above said delaytransition temperature, and

means for creating near to said junction optical trapping centerscapable of undergoing saturable absorption in response to an opticalfield near the junction,

said creating means comprising a P region located on the P-side of saidjunction and separated therefrom by a distance less than 1.5 microns.

2. The device of claim 1 wherein said laser is a gallium arsenide laser.

3. The device of claim 2 wherein the depth of said junction is about 1.8microns and the depth of said P region is about 1.0 microns.

4. The device of claim 1 in combination with means for switching saiddevice from one stable state to another comprising means for varying theinjection current applied to said laser so as to alternately saturateand unsaturate said trapping centers.

5. The device of claim 4 wherein said means for creating opticaltrapping centers near to the junction of said laser comprises a P regionlocated within the P-side of said laser and near to only a portion ofsaid junction, said current varying means being sufficient to cause saidp region to undergo saturable absorption and the remaining portion ofsaid P-side to undergo gain saturation.

6 The device of claim 1 for use as a unitary optical logic deviceincluding said C.W. laser having a longitudinal cavity axis along whichradiation is emitted,

second and third independent junction lasers disposed transverse to thelongitudinal axis of said c.w. laser,

said trapping centers being located near to at least a portion of thejunction of said C.W. laser, said portion being common to at least oneof said independent lasers.

7. The device of claim 6 wherein the external radiation from said oneindependent laser adjacent to said common portion causes said centerswithin said portion to saturate, the external radiation of said otherindependent laser causing gain saturation in said remaining portion ofsaid laser.

8. The device of claim 7 for use as a nondestructive memory wherein theinjection current applied to said adjacent laser is adapted so that itlases only if said laser lases and said other independent laser lasesonly when said C.W. laser does not lase.

9. The device of claim 7 wherein saturation of said centers lowers thecurrent threshold l of said C.W. laser by an amount A1 and gainsaturation increases by A1,, the injection current I of said CW. laserbeing maintained such that (1,, M l and l is less than the smaller of land (I A1,; Al thus performing the logic function D E+F where D, E and Frefer, respectively, to the logic states of said C.W. laser, said oneindependent laser and said other independent laser.

10. The device of claim 7 wherein saturation of said center lowers thecurrent threshold 1 of said C.W. laser by an amount AI and gainsaturation increases it by Al the injection current I of said C.W. laserbeing maintained such that l (1,, Ai and greater than the larger of Iand (I AI Alp), thus performing the logic function D= E -F, where D, Eand F refer, respectively, to the logic states of said C.W. laser, saidone independent laser and said other independent laser.

11. The device of claim 6 wherein said trapping centers are locatedadjacent to the entire P-N junction of said C.W. laser and wherein theexternal radiation from one of said independent lasers lowers thecurrent threshold 1 of said C.W. laser by an amount Al whereas theexternal radiation from both of said independent lasers operatingsimultaneously lowers said threshold by an amount of about 2A].

12. The device of claim 11 for use as an optical OR gate comprisingmeans for applying to said C.W. laser injection current of magnitudebetween (1 AI and 1 13. The device of claim 11 for use as an optical ANDgate comprising means for applying to said C.W. laser an injectioncurrent of magnitude between (l1/ !A/) and (l Al).

UNITED STATES PATENT OFFICE (IERTWKCATE OF I CCRRECTlGN I Patent No.3,65 l, l97 Dated April l, 1972 lnventofls) John C. Dyme-nt, Thomas L.Paoli and Jose E. Ripper It is certified that error appears in the aboveidentified patentand that said Letters Patent are hereby corrected asshown below:

Column 2, line 5 after-"short change lo to line 55, change sec" to---sec--- I Column 3, line 3O,'de1ete "1O /om 800C for 3" and insert--lO /cm The diffusion step, when l I carried out at 800C for '3--Column 3, line 36, delete "lO /om layer" and insert i lo /m Under theseconditions the thickness of the P+ layer--'.

Column 6, line 10, change "A 323C" to --A BC-.

I Column 6, line 30, delete "(1 A AI and insert 2 I Column 8, line 18,insert an end parenthesis after "AI" Column 8, line 21, -after "between"delete "(1 A and th 2 insert --(I,

? Signed and sealed this 7th day of November 1972.

(SEAL) Attest:

EDWARD M.FLETCI-IER,JR. l ROBERT GOTT CHALK Attesting OfficerCommissioner of Patents

1. An optical device comprising: a continuous wave P-N junction laserhaving a characteristic delay transition temperature, means formaintaining the temperature of said junction above said delay transitiontemperature, and means for creating near to said junction opticaltrapping centers capable of undergoing saturable absorption in responseto an optical field near the junction, said creating means comprising aP region located on the P-side of said junction and separated therefromby a distance less than 1.5 microns.
 2. The device of claim 1 whereinsaid laser is a gallium arsenide laser.
 3. The device of claim 2 whereinthe depth of said junction is about 1.8 microns and the depth of said Pregion is about 1.0 microns.
 4. The device of claim 1 in combinationwith means for switching said device from one stable state to anothercomprising means for varying the injection current applied to said laserso as to alternately saturate and unsaturate said trapping centers. 5.The device of claim 4 wherein said means for creating optical trappingcenters near to the junction of said laser comprises a P region locatedwithin the P-side of said laser and near to only a portion of saidjunction, said current varying means being sufficient to cause said pregion to undergo saturable absorption and the remaining portion of saidP-side to undergo gain saturation.
 6. The device of claim 1 for use as aunitary optical logic device including said C.W. laser having alongitudinal cavity axis along which radiation is emitted, second andthird independent junction lasers disposed transverse to thelongitudinal axis of said c.w. laser, said trapping centers beinglocated near to at least a portion of the junction of said C.W. laser,said portion being common to at least one of said independent lasers. 7.The device of claim 6 wherein the external radiation from said oneindependent laser adjacent to said common portion causes said centerswithin said portion to saturate, the external radiation of said otherindependent laser causing gain saturation in said remaining portion ofsaid laser.
 8. The device of claim 7 for use as a nondestructive memorywherein the injection current applied to said adjacent laser is adaptedso that it lases only if said laser lases and said other independentlaser lases only when said C.W. laser does not lase.
 9. The device ofclaim 7 wherein saturation of said centers lowers the current thresholdIth of said C.W. laser by an amount Delta IE and gain saturationincreases by Delta IF, the injection current I of said C.W. laser beingmaintained such that (Ith - Delta IE) < I and I is less than the smallerof Ith and (Ith - Delta IE + Delta IF), thus performing the logicfunction D E+F, where D, E and F refer, respectively, to the logicstates of said C.W. laser, said one independent laser and said otherindependent laser.
 10. The device of claim 7 wherein saturation of saidcenter lowers the current threshold Ith of said C.W. laser by an amountDelta IE and gain saturation increases it by Delta IF, the injectioncurrent I of said C.W. laser being maintained such that I < (Ith + DeltaIF) and greater than the larger of Ith and (Ith - Delta IE + Delta IF),thus performing the logic function D E .F, where D, E and F refer,respectively, to the logic states of said C.W. laser, said oneindependent laser and said other independent laser.
 11. The device ofclaim 6 wherein said trapping centers are located adjacent to the entireP-N junction of said C.W. laser and wherein the external radiation fromone of said independent lasers lowers the current threshold Ith of saidC.W. laser by an amount Delta I whereas the external radiation from bothof said independent lasers operating simultaneously lowers saidthreshold by an amount of about 2 Delta I.
 12. The device of claim 11for use as an optical OR gate comprising means for applying to said C.W.laser injection current of magnitude between (Ith - Delta I and Ith. 13.The device of claim 11 for use as an optical AND gate comprising meansfor applying to said C.W. laser an injection current of magnitudebetween (Ith - 2 Delta I) and (Ith - Delta I).